Starting in approximately 1984 low cost personal laser printers became available. All dry electrophotographic copiers and printers develop an image utilizing a dry toner. The typical toner is composed of styrene acrylic resin, a pigment-typically carbon black, and a charge control dye to endow the toner with the desired tribocharging properties for developing a latent electrostatic image. Styrene acrylic resin is a thermoplastic which can be melted and fused to the desired medium, typically paper.
The typical fusing system in an electrophotographic printer or copier is composed of two heated platen rollers which, when print media with a developed image pass between them, melt the toner and through pressure physically fuse the molten thermal plastic to the medium. Heating is usually accomplished by placing a high power tungsten filament quartz lamp inside the hollow platen roller.
Presently, most printer and copier fusing systems and their temperature control systems are not designed to compensate for differing media types or changes in thermal loading as a result of media passing through the printer or copier. The typical fusing system is designed with a heating element capable of providing enough heat to deal with all foreseen media with little or no concern to the resulting poor power quality that results. Some relatively new printers do utilize optical sensors to differentiate between paper and overhead transparencies. These additional sensors, which are being added to the printing mechanisms in order to improve image quality, can also be utilized by the fuser control systems to improve temperature regulation as well as improve the power quality of the overall printing system.
There are numerous reasons to intelligently control an electrophotographic printer or copier (herein referred to collectively as printers) fusing system in a much more aggressive manner. First, intelligent control can result in a universal fuser that can be shipped to any commercial market worldwide regardless of the power system. The universal fuser is a fusing system which can be connected to any low voltage public power system worldwide. Second, a flicker free universal fuser has the attractive benefit of requiring a single part for both manufacture and field service replacement. As used here, flicker is the visual perception of ambient light fluctuations within the home or office due to large transient power loads inducing voltage changes on the low voltage public power distribution system. The manufacturer is relieved of the burden of manufacturing 110 VAC and 220 VAC printers. The need to stock two types of service parts is eliminated, and product distribution centers now have one product that can be shipped to any country in the world without any reconfiguration requirements. There are reduced logistical burdens for sales, distribution and manufacture scheduling. As can be expected there is a large financial advantage to be gained by producing only a single version of a product for worldwide consumption.
For a dry electrophotographic fusing system to operate worldwide it must be able to operate satisfactorily on AC power systems providing from 90 Vrms to 240 Vrms at frequencies of 50 Hz to 60 Hz. The fusing system must heat up from ambient room temperature to operating temperature as quickly as possible while exhibiting extremely low flicker as its power consumption level changes. The fusing system, when combined with the balance of the electrophotographic printer power electronics, must meet International Electrical Commission (IEC) regulations IEC 555-2 and IEC 555-3 for current harmonics and flicker. The printer must pass Federal Communications Commission (FCC) class B regulations for power line conducted emissions and radiated emissions. In addition, the printer must pass CISPR B requirements for power line conducted emissions and radiated emissions. Finally, the printer must not suffer from excessive acoustic multi-tone or single tone emissions in the human auditory range in the office environment. The fusing system must be capable of switching into a power down or power off mode for energy savings as suggested by the EPA Energy Star Program. The absolute cost of any additional electronics is limited to no more than the cost benefit of not stocking multiple 110 VAC and 220 VAC models.
U.S. Pat. No. 5,483,149 to Barrett (herein referred to as Barrett) shows that a universal fuser may be obtained through the use of a modified integral half cycle (IHC) power controller but without solving the flicker problem at high power. The method taught by Barrett has been shown to suffer some flicker problems as well as placing current sub-harmonics on the AC power system. Currently no regulation exists regarding AC current sub-harmonic content.
Other methods such as phase control, in which a triac's conduction angle is ramped up relatively slowly, have proven to yield a universal fusing system which meets IEC 555-3 specifications. In U.S. Pat. No. 4,928,055 to Kaieda et al. (herein referred to as Kaieda) a fuser power control system based on phase delay gated triac control of an AC heating system is taught.
Prior to the present invention, most printers relied on the large thermal mass of the fusing system to average out the temperature changes induced by the media as it passes through the fusing system. Use of a large thermal mass allows a simple proportional temperature controller to maintain the average temperature of the fuser near the desired temperature. However, a large thermal mass takes longer to heat up, which increases the time for first page out when first powering up the printer or after exiting a power save mode. Additionally, the large thermal mass increases the physical weight and cost of the printer and the replacement parts. Finally, the large thermal mass also experiences a large pure time delay which causes temperature controller to oscillate.
Most commercially implemented fuser temperature controllers utilized triac based power controllers controlled by an oscillating proportional temperature controller. These systems generate high levels of flicker. The oscillatory nature of the temperature control systems coupled with the large phase lag in the system induced large temperature swings in the fuser temperature as media traveled through the fuser platens. These systems tended to run the fuser system at a very high temperature in order to attain adequate fusing of toner to heavy paper media at high relative humidity. When there prior systems where oscillating the power to the fuser certain portions of the paper media would receive not enough energy to completely fuse the toner to the paper.
The typical temperature controller drives a triac based power controller. The triac based power controller provides ease of implementation at a low cost. The triac based system may be controlled by an integral half cycle controller (IHC) in which a number of half cycles of AC power are supplied to the fusing system and a number of half cycles of no power are supplied to the fuser. The ratio of the number of power cycles to the total of the power cycles plus the non-power cycles is called the duty cycle, or duty ratio, of the IHC controller. A triac based system may also be controlled by a phase control system which allows the triac to supply power to the fuser for discrete portions of the AC half cycle. The portion of the AC cycle in which the triac is conducting is known as the conduction angle.